The present invention relates to apparatus for manipulating sheets, and more particularly to improvements in apparatus for manipulating folded sheets. Still more particularly, the invention relates to apparatus for singularizing folded sheets which form a stack and must be opened up or spread apart for further processing in a bookbinding or like machine, and wherein one marginal portion of one of the two panels of each sheet projects beyond the corresponding marginal portion of the other panel.
As a rule, folded sheets of the above outlined character are processed in an apparatus wherein a continuously rotating drum withdraws successive sheets from a magazine and positions the aforementioned marginal portions of the two panels of each withdrawn sheet in the path of movement of opening or spreading drums which are provided with clamping or gripping devices. The clamping device or devices of one spreading drum engage the projecting marginal portion, and the clamping device or devices of the other spreading drum engage the other marginal portion prior to movement of the clamping devices away from each other to thus open the sheet preparatory to transfer onto a conveyor which delivers the sheets to a further processing station. The clamping device of devices of the one spreading drum engage the respective (projecting) marginal portion before the other marginal portion is engaged by the clamping device or devices of the other spreading drum.
Such apparatus operate properly as long as the marginal portions (especially the projecting marginal portions) of the panels which form part of withdrawn sheets are not deformed, e.g., folded, curled or otherwise moved out of requisite or anticipated position. The likelihood that the projecting marginal portion will undergo undesirable deformation which prevents the corresponding clamping device or devices from engaging the deformed projecting marginal portion is especially pronounced when the sheets are relatively thin and readily flexible. Failure of the clamping device or devices of the one spreading drum to engage a deformed projecting marginal portion results in unpredictable transport of the respective sheet and causes lengthy interruptions of operation.